Two distinct types of Langerhans cells populate the skin during steady state and inflammation

Smad2/4 Signaling Pathway Is Critical for Epidermal Langerhans Cell Repopulation Under Inflammatory Condition but Not Required for Their Homeostasis at Steady State

Epidermal Langerhans cells (LCs) are skin-resident dendritic cells that are essential for the induction of skin immunity and tolerance. Transforming growth factor-β 1 (TGFβ1) is a crucial factor for LC maintenance and function. However, the underlying TGFβ1 signaling pathways remain unclear. Our previous research has shown that the TGFβ1/Smad3 signaling pathway does not impact LC homeostasis and maturation. In this study, we generated mice with conditional deletions of either individual Smad2, Smad4, or both Smad2 and Smad4 in the LC lineage or myeloid lineage, to further explore the impact of TGFβ1/Smad signaling pathways on LCs. We found that interruption of Smad2 or Smad4 individually or simultaneously in the LC lineage did not significantly impact the maintenance, maturation, antigen uptake, and migration of LCs in vivo or in vitro during steady state. However, the interruption of both Smad2 and Smad4 pathways in the myeloid lineage led to a dramatic inhibition of bone marrow-derived LCs in the inflammatory state. Overall, our data suggest that canonical TGFβ1/Smad2/4 signaling pathways are dispensable for epidermal LC homeostasis and maturation at steady state, but are critical for the long-term LC repopulation directly originating from the bone marrow in the inflammatory state.

AAAPT Diagnostic Criteria for Acute Knee Arthroplasty Pain

OBJECTIVE

The relationship between preexisting osteoarthritic pain and subsequent post-total knee arthroplasty (TKA) pain is not well defined. This knowledge gap makes diagnosis of post-TKA pain and development of management plans difficult and may impair future investigations on personalized care. Therefore, a set of diagnostic criteria for identification of acute post-TKA pain would inform standardized management and facilitate future research.

METHODS

The Analgesic, Anesthetic, and Addiction Clinical Trial Translations, Innovations, Opportunities, and Networks (ACTTION) public-private partnership with the US Food and Drug Administration (FDA), the American Pain Society (APS), and the American Academy of Pain Medicine (AAPM) formed the ACTTION-APS-AAPM Pain Taxonomy (AAAPT) initiative to address this goal. A multidisciplinary work group of pain experts was invited to conceive diagnostic criteria and dimensions of acute post-TKA pain.

RESULTS

The working group used contemporary literature combined with expert opinion to generate a five-dimensional taxonomical structure based upon the AAAPT framework (i.e., core diagnostic criteria, common features, modulating factors, impact/functional consequences, and putative mechanisms) that characterizes acute post-TKA pain.

CONCLUSIONS

The diagnostic criteria created are proposed to define the nature of acute pain observed in patients following TKA.

The developing immune network in human prenatal skin

Establishment of a well‐functioning immune network in skin is crucial for its barrier function. This begins in utero alongside the structural differentiation and maturation of skin, and continues to expand and diversify across the human lifespan. The microenvironment of the developing human skin supports immune cell differentiation and has an overall anti‐inflammatory profile. Immunologically inert and skewed immune populations found in developing human skin promote wound healing, and as such may play a crucial role in the structural changes occurring during skin development.

Altered Monocyte and Langerhans Cell Innate Immunity in Patients With Recurrent Respiratory Papillomatosis (RRP)

The micromilieu within respiratory papillomas supports persistent human papillomavirus (HPV) infection and disease recurrence in patients with recurrent respiratory papillomatosis (RRP). These patients show polarized (T_H2-/Treg) adaptive immunity in papillomas and blood, enriched immature Langerhans cell (iLC) numbers, and overexpression of cyclooxygenase-2/prostaglandin E₂ (PGE₂) in the upper airway. Blood monocyte-derived, and tissue-derived iLCs from RRP patients and controls were now studied to more fully understand innate immune dysregulation in RRP. Patients' monocytes generated fewer iLCs than controls, due to a reduced fraction of classical monocytes that generated most but not all the iLCs. Prostaglandin E₂, which was elevated in RRP plasma, reduced monocyte-iLC differentiation from controls to the levels of RRP patients, but had no effect on subsequent iLC maturation. Cytokine/chemokine responses by iLCs from papillomas, foreskin, and abdominal skin differed significantly. Freshly derived tissue iLCs expressed low CCL-1 and high CCL-20 mRNAs and were unresponsive to IL-36γ stimulation. Papilloma iLCs uniquely expressed IL-36γ at baseline and expressed CCL1 when cultured overnight outside their immunosuppressive microenvironment without additional stimulation. We conclude that monocyte/iLC innate immunity is impaired in RRP, in part due to increased PGE₂ exposure in vivo. The immunosuppressive papilloma microenvironment likely alters iLC responses, and vice versa, supporting T_H2-like/Treg HPV-specific adaptive immunity in RRP.

BMP7 aberrantly induced in the psoriatic epidermis instructs inflammation-associated Langerhans cells

BACKGROUND

Epidermal hyperplasia represents a morphologic hallmark of psoriatic skin lesions. Langerhans cells (LCs) in the psoriatic epidermis engage with keratinocytes (KCs) in tight physical interactions; moreover, they induce T-cell-mediated immune responses critical to psoriasis.

OBJECTIVE

This study sought to improve the understanding of epidermal factors in psoriasis pathogenesis.

METHODS

BMP7-LCs versus TGF-β1-LCs were phenotypically characterized and their functional properties were analyzed using flow cytometry, cell kinetic studies, co-culture with CD4 T cells, and cytokine measurements. Furthermore, immunohistology of healthy and psoriatic skin was performed. Additionally, in vivo experiments with Jun^f/fJunB^f/fK5cre-ER^T mice were carried out to assess the role of bone morphogenetic protein (BMP) signaling in psoriatic skin inflammation.

RESULTS

This study identified a KC-derived signal (ie, BMP signaling) to promote epidermal changes in psoriasis. Whereas BMP7 is strictly confined to the basal KC layer in the healthy skin, it is expressed at high levels throughout the lesional psoriatic epidermis. BMP7 instructs precursor cells to differentiate into LCs that phenotypically resemble psoriatic LCs. These BMP7-LCs exhibit proliferative activity and increased sensitivity to bacterial stimulation. Moreover, aberrant high BMP signaling in the lesional epidermis is mediated by a KC intrinsic mechanism, as suggested from murine data and clinical outcome after topical antipsoriatic treatment in human patients.

CONCLUSIONS

These data indicate that available TGF-β family members within the lesional psoriatic epidermis preferentially signal through the canonical BMP signaling cascade to instruct inflammatory-type LCs and to promote psoriatic epidermal changes. Targeting BMP signaling might allow to therapeutically interfere with cutaneous psoriatic manifestations.

Dendritic cells: The driver of psoriasis

Psoriasis is a chronic skin inflammatory disorder, the immune mechanism of which has been profoundly elucidated in the past few years. The dominance of the interleukin (IL)-23/IL-17 axis is a significant breakthrough in the understanding of the pathogenesis of psoriasis, and treatment targeting IL-23 and IL-17 has successfully benefited patients with the disease. The skin contains a complex network of dendritic cells (DC) mainly composed of epidermal Langerhans cells, bone marrow-derived dermal conventional DC, plasmacytoid DC and inflammatory DC. As the prominent cellular source of α-interferon, tumor necrosis factor-α, IL-12 and IL-23, DC play a pivotal role in psoriasis. Thus, targeting pathogenic DC subsets is a valid strategy for alleviating and preventing psoriasis and other DC-derived diseases. In this review, we survey the known role of DC in this disease.

Transcriptional control of dendritic cell development and functions

Dendritic cells (DCs) are major regulators of adaptive immunity, as they are not only capable to induce efficient immune responses, but are also crucial to maintain peripheral tolerance and thereby inhibit autoimmune reactions. DCs bridge the innate and the adaptive immune system by presenting peptides of self and foreign antigens as peptide MHC complexes to T cells. These properties render DCs as interesting target cells for immunomodulatory therapies in cancer, but also autoimmune diseases. Several subsets of DCs with special properties and functions have been described. Recent achievements in understanding transcriptional programs on single cell level, together with the generation of new murine models targeting specific DC subsets, advanced our current understanding of DC development and function. Thus, DCs arise from precursor cells in the bone marrow with distinct progenitor cell populations splitting the monocyte populations and macrophage populations from the DC lineage, which upon lineage commitment can be separated into conventional cDC1, cDC2, and plasmacytoid DCs (pDCs). The DC populations harbor intrinsic programs enabling them to react for specific pathogens in dependency on the DC subset, and thereby orchestrate T cell immune responses. Similarities, but also varieties, between human and murine DC subpopulations are challenging, and will require further investigation of human specimens under consideration of the influence of the tissue micromilieu and DC subset localization in the future.

The Impact of Versatile Macrophage Functions on Acute Kidney Injury and Its Outcomes

Acute kidney injury (AKI) is a common and devastating clinical condition with a high morbidity and mortality rate and is associated with a rapid decline of kidney function mostly resulting from the injury of proximal tubules. AKI is typically accompanied by inflammation and immune activation and involves macrophages (Mϕ) from the beginning: The inflamed kidney recruits “classically” activated (M1) Mϕ, which are initially poised to destroy potential pathogens, exacerbating inflammation. Of note, they soon turn into “alternatively” activated (M2) Mϕ and promote immunosuppression and tissue regeneration. Based on their roles in kidney recovery, there is a growing interest to use M2 Mϕ and Mϕ-modulating agents therapeutically against AKI. However, it is pertinent to note that the clinical translation of Mϕ-based therapies needs to be critically reviewed and questioned since Mϕ are functionally plastic with versatile roles in AKI and some Mϕ functions are detrimental to the kidney during AKI. In this review, we discuss the current state of knowledge on the biology of different Mϕ subtypes during AKI and, especially, on their role in AKI and assess the impact of versatile Mϕ functions on AKI based on the findings from translational AKI studies.

The role of Langerhans cells in pathologies of the skin

Langerhans cells (LCs) are epidermal immune cells of myeloid origin. Although these cells were primarily thought to play a defensive role in the skin, evidence now indicates a diverse range of LC-mediated effects including the relay of viral antigens in herpes simplex infection, recruitment of eosinophils in atopic dermatitis and promotion of a Th17 response in Candida infection. LCs may have a protective or suppressive function in pathologies of the skin, with differing functions being driven by the skin milieu. Understanding LC function will help guide the development of interventions that modulate these cells for therapeutic benefit.

The Dynamics of the Skin's Immune System

The skin is a complex organ that has devised numerous strategies, such as physical, chemical, and microbiological barriers, to protect the host from external insults. In addition, the skin contains an intricate network of immune cells resident to the tissue, crucial for host defense as well as tissue homeostasis. In the event of an insult, the skin-resident immune cells are crucial not only for prevention of infection but also for tissue reconstruction. Deregulation of immune responses often leads to impaired healing and poor tissue restoration and function. In this review, we will discuss the defensive components of the skin and focus on the function of skin-resident immune cells in homeostasis and their role in wound healing.

Ezh2 Controls Skin Tolerance through Distinct Mechanisms in Different Subsets of Skin Dendritic Cells

Ezh2, a well-established epigenetic repressor, can down-regulate leukocyte inflammatory responses, but its role in cutaneous health remains elusive. Here we demonstrate that Ezh2 controls cutaneous tolerance by regulating Langerhans cell (LC) transmigration across the epidermal basement membrane directly via Talin1 methylation. Ezh2 deficiency impaired disassembly of adhesion structures in LCs, leading to their defective integrin-dependent emigration from the epidermis and failure in tolerance induction. Moreover, mobilization of Ezh2-deficient Langerin^– dermal dendritic cells (dDCs) via high-dose treatment with a weak allergen restored tolerance, which is associated with an increased tolerogenic potential of Langerin^– dDCs likely due to epigenetic de-repression of Aldh in the absence of Ezh2. Our data reveal novel roles for Ezh2 in governing LC- and dDC-mediated host protection against cutaneous allergen via distinct mechanisms.

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Ezh2 regulates LC transmigration across basement membrane via Talin1 methylation

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Ezh2-mediated LC migration is required for cutaneous tolerance induction

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Ezh2 represses Aldh epigenetically in dermal DCs

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Ezh2-deficient dermal DCs exhibit increased tolerogenicity

RARα supports the development of Langerhans cells and langerin-expressing conventional dendritic cells

Langerhans cells (LC) are the prototype langerin-expressing dendritic cells (DC) that reside specifically in the epidermis, but langerin-expressing conventional DCs also reside in the dermis and other tissues, yet the factors that regulate their development are unclear. Because retinoic acid receptor alpha (RARα) is highly expressed by LCs, we investigate the functions of RARα and retinoic acid (RA) in regulating the langerin-expressing DCs. Here we show that the development of LCs from embryonic and bone marrow-derived progenitors and langerin⁺ conventional DCs is profoundly regulated by the RARα-RA axis. During LC differentiation, RARα is required for the expression of a LC-promoting transcription factor Runx3, but suppresses that of LC-inhibiting C/EBPβ. RARα promotes the development of LCs and langerin⁺ conventional DCs only in hypo-RA conditions, a function effectively suppressed at systemic RA levels. Our findings identify positive and negative regulatory mechanisms to tightly regulate the development of the specialized DC populations.

Langerhans cells (LC) and langerin-expressing conventional dendritic cells are made from distinct progenitors and enriched in the distinct microenvironments of the skin. Here the authors show that these immune cells are regulated by retinoic acid receptor alpha (RARα) via simultaneous induction of LC-promoting Runx3 and repression of LC-inhibiting C/EBPβ.

The role of inhibitor of binding or differentiation 2 in the development and differentiation of immune cells

Inhibitor of binding or differentiation 2 (Id2), a member of helix-loop-helix (HLH) transcriptional factors, is recently reported as an important regulator of the development or differentiation of immune cells. It has been demonstrated that Id2 plays a critical role in the early lymphopoiesis. However, it has been discovered recently that Id2 displays new functions in different immune cells. In the adaptive immune cells, Id2 is required for determining T-cell subsets and B cells. In addition, Id2 is also involved in the development of innate immune cells, including dendritic cells (DCs), natural killer (NK) cells, and other innate lymphoid cells (ILCs). Here, we review the current reports about the role of Id2 in the development or differentiation of main immune cells.

Uncovering the Mysteries of Langerhans Cells, Inflammatory Dendritic Epidermal Cells, and Monocyte-Derived Langerhans Cell-Like Cells in the Epidermis

The identity of Langerhans cells (LCs) has been called into question of late due to the increasing evidence that LCs originate from macrophage lineage instead of dendritic cell (DC) lineage as previously thought. For many years, LCs have been assumed to be DCs due to its migratory capabilities. However, recent studies have demonstrated that LCs are from macrophage lineage of the adult fetal liver (FL) progenitor. Bona fide LCs are now considered tissue-resident macrophages as they originate from the FL as shown by fate mapping models. In recent years, studies have shown that there are three types of antigen-presenting cells present in the epidermis, such as LCs, monocyte-derived LC-like cells, and inflammatory dendritic epidermal cells (IDECs). Of these, LC-like cells have been characterized in both human and mouse studies, while IDECs have only been described in human studies. This has shed a new light on the area of epidermal macrophages, suggesting that there might be a misidentification and misclassification of LCs. IDECs and LC-like cells have been shown to be present in both steady state and inflammatory state, but they are present in more significant amounts under inflammatory conditions such as atopic dermatitis, ultra violet injury, and psoriasis. In this review, we discuss what is already known and discuss the possible roles of LCs, LC-like cells, and IDECs during inflammation. Most intriguingly, we discuss the possibility of LCs having a dual identity as both a macrophage and a DC. This is shown as LCs are the only tissue-resident macrophage to have shown migratory property-like DCs.

Cell-intrinsic regulation of murine epidermal Langerhans cells by protein S

AXL, a member of the TYRO3, AXL, and MERTK (TAM) receptor tyrosine kinase family, has been shown to play a role in the differentiation and activation of epidermal Langerhans cells (LCs). Here, we demonstrate that growth arrest-specific 6 (GAS6) protein, the predominant ligand of AXL, has no impact on LC differentiation and homeostasis. We thus examined the role of protein S (PROS1), the other TAM ligand acting primarily via TYRO3 and MERTK, in LC function. Genetic ablation of PROS1 in keratinocytes resulted in a typical postnatal differentiation of LCs; however, a significant reduction in LC frequencies was observed in adult mice due to increased apoptosis. This was attributed to altered expression of cytokines involved in LC development and tissue homeostasis within keratinocytes. PROS1 was then excised in LysM⁺ cells to target LCs at early embryonic developmental stages, as well as in adult monocytes that also give rise to LCs. Differentiation and homeostasis of LCs derived from embryonic precursors was not affected following Pros1 ablation. However, differentiation of LCs from bone marrow (BM) precursors in vitro was accelerated, as was their capability to reconstitute epidermal LCs in vivo. These reveal an inhibitory role for PROS1 on BM-derived LCs. Collectively, this study highlights a cell-specific regulation of LC differentiation and homeostasis by TAM signaling.

Human dendritic cell subsets: an update

Dendritic cells (DC) are a class of bone-marrow-derived cells arising from lympho-myeloid haematopoiesis that form an essential interface between the innate sensing of pathogens and the activation of adaptive immunity. This task requires a wide range of mechanisms and responses, which are divided between three major DC subsets: plasmacytoid DC (pDC), myeloid/conventional DC1 (cDC1) and myeloid/conventional DC2 (cDC2). Each DC subset develops under the control of a specific repertoire of transcription factors involving differential levels of IRF8 and IRF4 in collaboration with PU.1, ID2, E2-2, ZEB2, KLF4, IKZF1 and BATF3. DC haematopoiesis is conserved between mammalian species and is distinct from monocyte development. Although monocytes can differentiate into DC, especially during inflammation, most quiescent tissues contain significant resident populations of DC lineage cells. An extended range of surface markers facilitates the identification of specific DC subsets although it remains difficult to dissociate cDC2 from monocyte-derived DC in some settings. Recent studies based on an increasing level of resolution of phenotype and gene expression have identified pre-DC in human blood and heterogeneity among cDC2. These advances facilitate the integration of mouse and human immunology, support efforts to unravel human DC function in vivo and continue to present new translational opportunities to medicine.

Control of mechanical pain hypersensitivity in mice through ligand-targeted photoablation of TrkB-positive sensory neurons

Mechanical allodynia is a major symptom of neuropathic pain whereby innocuous touch evokes severe pain. Here we identify a population of peripheral sensory neurons expressing TrkB that are both necessary and sufficient for producing pain from light touch after nerve injury in mice. Mice in which TrkB-Cre-expressing neurons are ablated are less sensitive to the lightest touch under basal conditions, and fail to develop mechanical allodynia in a model of neuropathic pain. Moreover, selective optogenetic activation of these neurons after nerve injury evokes marked nociceptive behavior. Using a phototherapeutic approach based upon BDNF, the ligand for TrkB, we perform molecule-guided laser ablation of these neurons and achieve long-term retraction of TrkB-positive neurons from the skin and pronounced reversal of mechanical allodynia across multiple types of neuropathic pain. Thus we identify the peripheral neurons which transmit pain from light touch and uncover a novel pharmacological strategy for its treatment.

There are several classes of sensory neuron that contribute to pain states. Here, the authors demonstrate that TrkB⁺ sensory neurons detect light touch under normal conditions in mice but contribute to hypersensitivity in models of chronic pain, and that ligand-guided laser ablation of TrkB⁺ sensory neurons in the mouse skin attenuates this hypersensitivity.

PPARγ Contributes to Immunity Induced by Cancer Cell Vaccines That Secrete GM-CSF

Peroxisome proliferator activated receptor-γ (PPARγ) is a lipid-activated nuclear receptor that promotes immune tolerance through effects on macrophages, dendritic cells (DCs), and regulatory T cells (Tregs). Granulocyte-macrophage colony stimulating factor (GM-CSF) induces PPARγ expression in multiple myeloid cell types. GM-CSF contributes to both immune tolerance and protection, but the role of PPARγ in these pathways is poorly understood. Here, we reveal an unexpected stimulatory role for PPARγ in the generation of antitumor immunity with irradiated, GM-CSF-secreting tumor-cell vaccines (GVAX). Mice harboring a deletion of pparg in lysozyme M (LysM)-expressing myeloid cells (KO) showed a decreased ratio of CD8⁺ T effectors to Tregs and impaired tumor rejection with GVAX. Diminished tumor protection was associated with altered DC responses and increased production of the Treg attracting chemokines CCL17 and CLL22. Correspondingly, the systemic administration of PPARγ agonists to vaccinated mice elevated the CD8⁺ T effector to Treg ratio through effects on myeloid cells and intensified the antitumor activity of GVAX combined with cytotoxic T lymphocyte-associated antigen-4 antibody blockade. PPARγ agonists similarly attenuated Treg induction and decreased CCL17 and CCL22 levels in cultures of human peripheral blood mononuclear cells with GM-CSF-secreting tumor cells. Together, these results highlight a key role for myeloid cell PPARγ in GM-CSF-stimulated antitumor immunity and suggest that PPARγ agonists might be useful in cancer immunotherapy. Cancer Immunol Res; 6(6); 723-32. ©2018 AACR.

Resident and monocyte-derived Langerhans cells are required for imiquimod-induced psoriasis-like dermatitis model

BACKGROUND

Langerhans cells (LCs) are dendritic cells that reside in the epidermis and local inflammation results in an increased differentiation of monocyte-derived LCs. Only few studies have investigated on the role of LCs in psoriasis-like dermatitis model, but the results are variable and the exact role of LCs in psoriasis model remains to be elucidated.

OBJECTIVE

To explore the functional role of resident (rLCs) and monocyte-derived LCs (mLCs) in imiquimod (IMQ)-induced psoriasis-like inflammation using human Langerin-diphtheria toxin subunit A (huLang-DTA) mice.

METHODS

5% IMQ cream was topically applied on the skins. Clinical and histopathological features were evaluated. Psoriasis-related gene expression was analyzed by quantitative polymerase chain reaction. The production of psoriasis-related cytokines including IL-17A and IL-22 by T cells were assessed by flow cytometry from the lesional skins.

RESULTS

huLang-DTA mice showed a common depletion of both rLCs and mLCs in the IMQ-treated skins. huLang-DTA mice had a reduced IMQ-induced psoriasis-like inflammation featuring erythema, scale, and thickness compared with wild-type mice. Psoriatic lesions from huLang-DTA mice had a decreased level of Il23a and accordingly demonstrated an attenuated cytokine production of IL-17A and IL-22 from γδ^low T cells. mLCs revealed a significantly greater level of IL-23 expression compared to rLCs in response to topical IMQ treatment.

CONCLUSION

Although both rLCs and mLCs are involved in the development of IMQ-induced psoriasis-like dermatitis, inflammation-induced mLCs present a superior capacity for producing IL-23 in this murine experimental model of psoriasis.

Adjuvant selection regulates gut migration and phenotypic diversity of antigen-specific CD4+ T cells following parenteral immunization

Infectious diarrheal diseases are the second leading cause of death in children under 5 years, making vaccines against these diseases a high priority. It is known that certain vaccine adjuvants, chiefly bacterial ADP-ribosylating enterotoxins, can induce mucosal antibodies when delivered parenterally. Based on this, we reasoned vaccine-specific mucosal cellular immunity could be induced via parenteral immunization with these adjuvants. Here, we show that, in contrast to the Toll-like receptor-9 agonist CpG, intradermal immunization with non-toxic double-mutant heat-labile toxin (dmLT) from enterotoxigenic Escherichia coli drove endogenous, antigen-specific CD4⁺ T cells to expand and upregulate the gut-homing integrin α4β7. This was followed by T-cell migration into gut-draining lymph nodes and both small and large intestines. We also found that dmLT produces a balanced T-helper 1 and 17 (Th1 and Th17) response, whereas T cells from CpG immunized mice were predominantly Th1. Immunization with dmLT preferentially engaged CD103⁺ dendritic cells (DCs) compared with CpG, and mice deficient in CD103⁺ DCs were unable to fully license antigen-specific T-cell migration to the intestinal mucosae following parenteral immunization. This work has the potential to redirect the design of existing and next generation vaccines to elicit pathogen-specific immunity in the intestinal tract with non-mucosal immunization.

Micro-environmental signals directing human epidermal Langerhans cell differentiation

Human Langerhans cells (LC) can be generated ex vivo from hematopoietic precursor cells in response to cytokines and cell-membrane associated ligands. These in vitro differentiation models provided mechanistic insights into the molecular and cellular pathways underlying the development of this unique, epithelia-associated dendritic cell subset. Notably, the human epidermal microenvironment is fully sufficient to induce LC differentiation from hematopoietic progenitors. Hence, dissecting the molecular characteristics of the human epithelial/epidermal LC niche, and testing defined ligands for their capacity to induce LC differentiation, led to a refined molecular model of LC lineage commitment. During epidermal ontogeny, spatially and temporally regulated availability of TGF-β family members cooperate with other keratinocyte-derived signals, such as E-cadherin and Notch ligands, for instructing LC differentiation. In this review, we discuss the signals known to instruct human hematopoietic progenitor cells and myelomonocytic cells to undergo LC lineage commitment. Additionally, the current methods for generation of large numbers of human LC-like cells ex vivo in defined serum-free media are discussed.

Human dendritic cell immunodeficiencies

The critical functions of dendritic cells (DCs) in immunity and tolerance have been demonstrated in many animal models but their non-redundant roles in humans are more difficult to probe. Human primary immunodeficiency (PID), resulting from single gene mutations, may result in DC deficiency or dysfunction. This relatively recent recognition illuminates the in vivo role of human DCs and the pathophysiology of the associated clinical syndromes. In this review, the development and function of DCs as established in murine models and human in vitro systems, discussed. This forms the basis of predicting the effects of DC deficiency in vivo and understanding the consequences of specific mutations on DC development and function. DC deficiency syndromes are associated with heterozygous GATA2 mutation, bi-allelic and heterozygous IRF8 mutation and heterozygous IKZF1 mutation. The intricate involvement of DCs in the balance between immunity and tolerance is leading to increased recognition of their involvement in a number of other immunodeficiencies and autoimmune conditions. Owing to the precise control of transcription factor gene expression by super-enhancer elements, phenotypic anomalies are relatively commonly caused by heterozygous mutations.

TIM-4 is differentially expressed in the distinct subsets of dendritic cells in skin and skin-draining lymph nodes and controls skin Langerhans cell homeostasis

T cell immunoglobulin and mucin-4 (TIM-4), mainly expressed on dendritic cells (DC) and macrophages, plays an essential role in regulating immune responses. Langerhans cells (LC), which are the sole DC subpopulation residing at the epidermis, are potent mediators of immune surveillance and tolerance. However, the significance of TIM-4 on epidermal LCs, along with other cutaneous DCs, remains totally unexplored. For the first time, we discovered that epidermal LCs expressed TIM-4 and displayed an increased level of TIM-4 expression upon migration. We also found that dermal CD207⁺ DCs and lymph node (LN) resident CD207⁻CD4⁺ DCs highly expressed TIM-4, while dermal CD207⁻ DCs and LN CD207⁻CD4⁻ DCs had limited TIM-4 expressions. Using TIM-4-deficient mice, we further demonstrated that loss of TIM-4 significantly upregulated the frequencies of epidermal LCs and LN resident CD207⁻CD4⁺ DCs. In spite of this, the epidermal LCs of TIM-4-deficient mice displayed normal phagocytic and migratory abilities, comparable maturation status upon the stimulation as well as normal repopulation under the inflamed state. Moreover, lack of TIM-4 did not affect dinitrofluorobenzene-induced contact hypersensitivity response. In conclusion, our results indicated that TIM-4 was differentially expressed in the distinct subsets of DCs in skin and skin-draining LNs, and specifically regulated epidermal LC and LN CD207⁻CD4⁺ DC homeostasis.

Langerhans Cells: Sensing the Environment in Health and Disease

In the last few decades, our understanding of Langerhans cells (LCs) has drastically changed based on novel findings regarding the developmental origin and biological functions of these epidermis-specific resident immune cells. It has become clear that LCs not only exert pivotal roles in immune surveillance and homeostasis but also impact on pathology by either inducing tolerance or mediating inflammation. Their unique capabilities to self-renew within the epidermis, while also being able to migrate to lymph nodes in order to present antigen, place LCs in a key position to sample the local environment and decide on the appropriate cutaneous immune response. Exciting new data distinguishing LCs from Langerin⁺ dermal dendritic cells (DCs) on a functional and ontogenic level reveal crucial roles for LCs in trauma and various skin pathologies, which will be thoroughly discussed here. However, despite rapid progress in the field, the exact role of LCs during immune responses has not been completely elucidated. This review focuses on what mouse models that have been developed in order to enable the study of murine LCs and other Langerin-expressing DCs have taught us about LC development and function.

Sequential BMP7/TGF-β1 signaling and microbiota instruct mucosal Langerhans cell differentiation

Capucha et al. demonstrate that mucosal Langerhans cell (LC) differentiation from pre–dendritic cells and monocytes involves consecutive BMP7 and TGF-β1 signaling in separate anatomical locations. Moreover, mucosal microbiota regulates the development of LCs that in turn shape microbial and immunological homeostasis.

Mucosal Langerhans cells (LCs) originate from pre–dendritic cells and monocytes. However, the mechanisms involved in their in situ development remain unclear. Here, we demonstrate that the differentiation of murine mucosal LCs is a two-step process. In the lamina propria, signaling via BMP7-ALK3 promotes translocation of LC precursors to the epithelium. Within the epithelium, TGF-β1 finalizes LC differentiation, and ALK5 is crucial to this process. Moreover, the local microbiota has a major impact on the development of mucosal LCs, whereas LCs in turn maintain mucosal homeostasis and prevent tissue destruction. These results reveal the differential and sequential role of TGF-β1 and BMP7 in LC differentiation and highlight the intimate interplay of LCs with the microbiota.

The Origin of Skin Dendritic Cell Network and Its Role in Psoriasis

Dendritic cells (DCs) are heterogeneous groups of innate immune cells, which orchestrate immune responses by presenting antigens to cognate T cells and stimulating other types of immune cells. Although the term ‘DCs’ generally represent highly mixed subsets with functional heterogeneity, the classical definition of DCs usually denotes conventional DCs (cDCs). Skin contains a unique DC network mainly composed of embryo precursor-derived epidermal Langerhans cells (LCs) and bone marrow-derived dermal cDCs, which can be further classified into type 1 (cDC1) and type 2 (cDC2) subsets. Psoriasis is a chronic inflammatory skin disease, which is principally mediated by IL-23/IL-17 cytokine axis. In the psoriatic skins, DCs are prominent cellular sources for TNF-α and IL-23, and the use of blocking antibodies against TNF-α and IL-23 leads to a significant clinical improvement in psoriatic patients. Recent elegant human and mouse studies have shown that inflammation-induced inflammatory DCs, LCs, dermal cDC2, and monocyte-derived DCs are pivotal DC subsets in psoriatic inflammation. Thus, targeting specific pathogenic DC subsets would be a potential strategy for alleviating and preventing DC-derived IL-23-dependent psoriatic inflammation and other inflammatory dermatoses in the future.

Impaired epidermal Langerhans cell maturation in TGFβ-inducible early gene 1 (TIEG1) knockout mice

TGF-β-inducible early gene 1 (TIEG1), also known as Krüppel-like factor 10 (Klf10), represents a major downstream transcription factor of transforming growth factor-β1 (TGF-β1) signaling. Epidermal Langerhans cells (LCs), a unique subpopulation of dendritic cells (DC), essentially mediates immune surveillance and tolerance. TGF-β1 plays a pivotal role in LC maintenance and function after birth, although the underpinning mechanisms remain elusive. Here, we hypothesized that TIEG1 might be involved in TGF-β1-mediated LC homeostasis and function. Utilizing TIEG1 null mice, we discovered that TIEG1 deficiency did not alter LC homeostasis at the steady state and LC repopulation at inflamed-state, as well as their antigen-uptake capacity, but significantly impaired their maturation ability, which was opposite to the fact that loss of TGF-β1 induced spontaneous LC maturation. Moreover, the ablation of TIEG1 enhanced skin contact hypersensitivity response. Our results suggested that TIEG1 is not a key molecule involved in TGF-β1-mediated homeostasis, while TIEG1-related signaling pathways regulate LC maturation and their function.

Langerhans Cells-Programmed by the Epidermis

Langerhans cells (LCs) reside in the epidermis as a dense network of immune system sentinels. These cells determine the appropriate adaptive immune response (inflammation or tolerance) by interpreting the microenvironmental context in which they encounter foreign substances. In a normal physiological, "non-dangerous" situation, LCs coordinate a continuous state of immune tolerance, preventing unnecessary and harmful immune activation. Conversely, when they sense a danger signal, for example during infection or when the physical integrity of skin has been compromised as a result of a trauma, they instruct T lymphocytes of the adaptive immune system to mount efficient effector responses. Recent advances investigating the molecular mechanisms underpinning the cross talk between LCs and the epidermal microenvironment reveal its importance for programming LC biology. This review summarizes the novel findings describing LC origin and function through the analysis of the transcriptomic programs and gene regulatory networks (GRNs). Review and meta-analysis of publicly available datasets clearly delineates LCs as distinct from both conventional dendritic cells (DCs) and macrophages, suggesting a primary role for the epidermal microenvironment in programming LC biology. This concept is further supported by the analysis of the effect of epidermal pro-inflammatory signals, regulating key GRNs in human and murine LCs. Applying whole transcriptome analyses and in silico analysis has advanced our understanding of how LCs receive, integrate, and process signals from the steady-state and diseased epidermis. Interestingly, in homeostasis and under immunological stress, the molecular network in LCs remains relatively stable, reflecting a key evolutionary need related to tissue localization. Importantly, to fulfill their key role in orchestrating antiviral adaptive immune responses, LC share specific transcriptomic modules with other DC types able to cross-present antigens to cytotoxic CD8⁺ T cells, pointing to a possible evolutionary convergence mechanism. With the development of more advanced technologies allowing delineation of the molecular networks at the level of chromatin organization, histone modifications, protein translation, and phosphorylation, future "omics" investigations will bring in-depth understanding of the complex molecular mechanisms underpinning human LC biology.

GM-CSF Monocyte-Derived Cells and Langerhans Cells As Part of the Dendritic Cell Family

Dendritic cells (DCs) and macrophages (Mph) share many characteristics as components of the innate immune system. The criteria to classify the multitude of subsets within the mononuclear phagocyte system are currently phenotype, ontogeny, transcription patterns, epigenetic adaptations, and function. More recently, ontogenetic, transcriptional, and proteomic research approaches uncovered major developmental differences between Flt3L-dependent conventional DCs as compared with Mphs and monocyte-derived DCs (MoDCs), the latter mainly generated in vitro from murine bone marrow-derived DCs (BM-DCs) or human CD14⁺ peripheral blood monocytes. Conversely, in vitro GM-CSF-dependent monocyte-derived Mphs largely resemble MoDCs whereas tissue-resident Mphs show a common embryonic origin from yolk sac and fetal liver with Langerhans cells (LCs). The novel ontogenetic findings opened discussions on the terminology of DCs versus Mphs. Here, we bring forward arguments to facilitate definitions of BM-DCs, MoDCs, and LCs. We propose a group model of terminology for all DC subsets that attempts to encompass both ontogeny and function.

Ontogeny and function of murine epidermal Langerhans cells

Langerhans cells (LCs) are epidermis-resident antigen-presenting cells that share a common ontogeny with macrophages but function as dendritic cells (DCs). Their development, recruitment and retention in the epidermis is orchestrated by interactions with keratinocytes through multiple mechanisms. LC and dermal DC subsets often show functional redundancy, but LCs are required for specific types of adaptive immune responses when antigen is concentrated in the epidermis. This Review will focus on those developmental and functional properties that are unique to LCs.

Targeting C-type lectin receptors: a high-carbohydrate diet for dendritic cells to improve cancer vaccines

There is a growing understanding of why certain patients do or do not respond to checkpoint inhibition therapy. This opens new opportunities to reconsider and redevelop vaccine strategies to prime an anticancer immune response. Combination of such vaccines with checkpoint inhibitors will both provide the fuel and release the brake for an efficient anticancer response. Here, we discuss vaccine strategies that use C-type lectin receptor (CLR) targeting of APCs, such as dendritic cells and macrophages. APCs are a necessity for the priming of antigen-specific cytotoxic and helper T cells. Because CLRs are natural carbohydrate-recognition receptors highly expressed by multiple subsets of APCs and involved in uptake and processing of Ags for presentation, these receptors seem particularly interesting for targeting purposes.

A type of human skin dendritic cell marked by CD5 is associated with the development of inflammatory skin disease

Dendritic cells (DCs) are important in regulating immunity and tolerance and consist of functionally distinct subsets that differentially regulate T lymphocyte function. The underlying basis for this subset specificity is lacking, particularly in humans, where the classification of tissue DCs is currently incomplete. Examination of healthy human epidermal Langerhans cells and dermal skin cells revealed a tissue CD5-expressing DC subtype. The CD5+ DCs were potent inducers of cytotoxic T cells and Th22 cells. The products of these T cells, IL-22 and IFN-γ, play a key role in the pathogenesis of psoriasis. Remarkably, CD5+ DCs were significantly enriched in lesional psoriatic skin compared with distal tissues, suggesting their involvement in the disease. We show that CD5+ DCs can be differentiated from hematopoietic progenitor cells independently of the CD5- DCs. A progenitor population found in human cord blood and in the dermal skin layer, marked as CD34-CD123+CD117dimCD45RA+, was an immediate precursor of these CD11c+CD1c+CD5+ DCs. Overall, our discovery of the CD5-expressing DC subtype suggests that strategies to regulate their composition or function in the skin will represent an innovative approach for the treatment of immune-mediated disorders in and beyond the skin.

Langerhans cell: exciting developments in health and disease

Langerhans cells (LCs) have been the subject of much research since their discovery in 1868. LCs belong to the subset of leucocytes called dendritic cells. They are present in the epidermis and the pilosebaceous apparatus and monitor the cutaneous environment for changes in homeostasis. During embryogenesis, a wave of yolk sac macrophages seed the fetal skin. Then, fetal liver monocytes largely replace the yolk sac macrophages and comprise the majority of adult LCs. In the presence of skin irritation, LCs process antigen and travel to regional lymph nodes to present antigen to reactive T lymphocytes. Changes in LCs' surface markers during the journey occur under the influence of cytokines. The difference in expression of surface markers and the ability to resist radiation have allowed researchers to differentiate LCs from the murine Langerin-positive dermal dendritic cells. Exciting discoveries have been made recently regarding their role in inflammatory skin diseases, cancer and HIV. New research has shown that antibodies blocking CD1a appear to mitigate inflammation in contact hypersensitivity reactions and psoriasis. While it has been established that LCs have the potential to induce effector cells of the adaptive immune system to counter oncogenesis, recent studies have demonstrated that LCs coordinate with natural killer cells to impair development of squamous cell carcinoma caused by chemical carcinogens. However, LCs may also physiologically suppress T cells and permit keratinocyte transformation and tumorigenesis. Although long known to play a primary role in the progression of HIV infection, it is now understood that LCs also possess the ability to restrict the progression of the disease. There is a pressing need to discover more about how these cells affect various aspects of health and disease; new information gathered thus far seems promising and exciting.

CD207+CD1a+ cells circulate in pediatric patients with active Langerhans cell histiocytosis

Langerhans cell histiocytosis (LCH) is a rare disease with an unknown etiology characterized by heterogeneous lesions containing CD207⁺CD1a⁺ cells that can arise in almost any tissue and cause significant morbidity and mortality. Precursors of pathological Langerhans cells have yet to be defined. Our aim was to identify circulating CD207⁺CD1a⁺ cells and their inducers in LCH. Expression of CD207 and CD1a in the blood myeloid compartment as well as thymic stromal lymphopoietin (TSLP) and transforming growth factor β (TGF-β) plasma levels were measured in 22 pediatric patients with active disease (AD) or nonactive disease (NAD). In patients with AD vs those with NAD, the myeloid compartment showed an increased CD11b (CD11b^high plus CD11b⁺) fraction (39.7 ± 3.6 vs 18.6 ± 1.9), a higher percentage of circulating CD11b^highCD11c⁺CD207⁺ cells (44.5 ± 11.3 vs 3.2 ± 0.5), and the presence of CD11c^highCD207⁺CD1a⁺ cells (25.0 ± 9.1 vs 2.3 ± 0.5). Blood CD207⁺CD1a⁺ cells were not observed in adult controls or umbilical cord. Increased TSLP and TGF-β levels were detected in patients with AD. Interestingly, plasma from patients with AD induces CD207 expression on CD14⁺ monocytes. We conclude that CD207⁺CD1a⁺ cells are circulating in patients with active LCH, and TSLP and TGF-β are potential drivers of Langerhans-like cells in vivo.

Monocyte, Macrophage, and Dendritic Cell Development: the Human Perspective

The maintenance of monocytes, macrophages, and dendritic cells (DCs) involves manifold pathways of ontogeny and homeostasis that have been the subject of intense study in recent years. The concept of a peripheral mononuclear phagocyte system continually renewed by blood-borne monocytes has been modified to include specialized DC pathways of development that do not involve monocytes, and longevity through self-renewal of tissue macrophages. The study of development remains difficult owing to the plasticity of phenotypes and misconceptions about the fundamental structure of hematopoiesis. However, greater clarity has been achieved in distinguishing inflammatory monocyte-derived DCs from DCs arising in the steady state, and new concepts of conjoined lymphomyeloid hematopoiesis more easily accommodate the shared lymphoid and myeloid phenotypes of some DCs. Cross-species comparisons have also yielded coherent systems of nomenclature for all mammalian monocytes, macrophages, and DCs. Finally, the clear relationships between ontogeny and functional specialization offer information about the regulation of immune responses and provide new tools for the therapeutic manipulation of myeloid mononuclear cells in medicine.

Cbfβ2 deficiency preserves Langerhans cell precursors by lack of selective TGFβ receptor signaling

Tenno et al. show that loss of Cbfβ2, one of two RNA splice variants of the Cbfb gene, results in the persistence of embryonic Langerhans cell precursors in the adult epidermis by selective loss of BMP7-BMPR1A signaling with intact TGFβR1 signaling.

The mouse Langerhans cell (LC) network is established through the differentiation of embryonic LC precursors. BMP7 and TGFβ1 initiate cellular signaling that is essential for inducing LC differentiation and preserving LCs in a quiescent state, respectively. Here we show that loss of Cbfβ2, one of two RNA splice variants of the Cbfb gene, results in long-term persistence of embryonic LC precursors after their developmental arrest at the transition into the EpCAM⁺ stage. This phenotype is caused by selective loss of BMP7-mediated signaling essential for LC differentiation, whereas TGFβR signaling is intact, maintaining cells in a quiescent state. Transgenic Cbfβ2 expression at the neonatal stage, but not at the adult stage, restored differentiation from Cbfβ2-deficient LC precursors. Loss of developmental potential in skin-residential precursor cells was accompanied by diminished BMP7–BMPR1A signaling. Collectively, our results reveal an essential requirement for the Cbfβ2 variant in LC differentiation and provide novel insight into how the establishment and homeostasis of the LC network is regulated.

Dual role of monocyte-derived dendritic cells in Trypanosoma cruzi infection

Pathogens can cause inflammation when inoculated into the skin. The vector-transmitted protozoan parasite Trypanosoma cruzi induces poor cellular-infiltration and disseminates, causing high mortality in the experimental model. Here, we characterized the inflammatory foci at the parasite inoculation site and secondary lymphoid organs using a murine model. While no macrophages and few neutrophils and monocytes (Mo) were recruited into the skin, T. cruzi infection elicited the mobilization of Ly6C⁺ Mo to draining lymph nodes and spleen. Over time, this population became enriched in CD11b⁺ Ly6C⁺ CD11c⁺ MHCII⁺ CD86⁺ cells resembling inflammatory dendritic cells (DCs). Adoptive transfer of Ly6C⁺ Mo purified from the bone marrow of CD11c-GFP transgenic mice confirmed the monocytic origin of Ly6C⁺ DCs found in the spleen of infected animals. Isolated Mo-derived cells not only produced TNF-α and nitric oxide, but also IL-10 and displayed a poor capacity to induce lymphoproliferation. Ablation of Mo-derived cells by 5-fluorouracil confirmed their dual role during infection, limiting the parasite load by inducible nitric oxide synthase-related mechanisms and negatively affecting the development of anti-parasite T-cell response. This study demonstrated that consistent with their antagonistic properties, these cells not only control the parasite spreading but also its persistence in the host.

Human skin dendritic cell fate is differentially regulated by the monocyte identity factor Kruppel-like factor 4 during steady state and inflammation

BACKGROUND

Langerhans cell (LC) networks play key roles in immunity and tolerance at body surfaces. LCs are established prenatally and can be replenished from blood monocytes. Unlike skin-resident dermal DCs (dDCs)/interstitial-type DCs and inflammatory dendritic epidermal cells appearing in dermatitis/eczema lesions, LCs lack key monocyte-affiliated markers. Inversely, LCs express various epithelial genes critical for their long-term peripheral tissue residency.

OBJECTIVE

Dendritic cells (DCs) are functionally involved in inflammatory diseases; however, the mechanisms remained poorly understood.

METHODS

In vitro differentiation models of human DCs, gene profiling, gene transduction, and immunohistology were used to identify molecules involved in DC subset specification.

RESULTS

Here we identified the monocyte/macrophage lineage identity transcription factor Kruppel-like factor 4 (KLF4) to be inhibited during LC differentiation from human blood monocytes. Conversely, KLF4 is maintained or induced during dermal DC and monocyte-derived dendritic cell/inflammatory dendritic epidermal cell differentiation. We showed that in monocytic cells KLF4 has to be repressed to allow their differentiation into LCs. Moreover, respective KLF4 levels in DC subsets positively correlate with proinflammatory characteristics. We identified epithelial Notch signaling to repress KLF4 in monocytes undergoing LC commitment. Loss of KLF4 in monocytes transcriptionally derepresses Runt-related transcription factor 3 in response to TGF-β1, thereby allowing LC differentiation marked by a low cytokine expression profile.

CONCLUSION

Monocyte differentiation into LCs depends on activation of Notch signaling and the concomitant loss of KLF4.

Regulation of Innate and Adaptive Immunity by TGFβ

Immune regulation by cytokines is crucial in maintaining immune homeostasis, promoting responses to infection, resolving inflammation, and promoting immunological memory. Additionally, cytokine responses drive pathology in immune-mediated disease. A crucial cytokine in the regulation of all aspects of an immune response is transforming growth factor beta (TGFβ). Although best known as a crucial regulator of T cell responses, TGFβ plays a vital role in regulating responses mediated by virtually every innate and adaptive immune cell, including dendritic cells, B cells, NK cells, innate lymphoid cells, and granulocytes. Here, we review our current knowledge of how TGFβ regulates the immune system, highlighting the multifunctional nature of TGFβ and how its function can change depending on location and context of action.

Development and Functional Differentiation of Tissue-Resident Versus Monocyte-Derived Macrophages in Inflammatory Reactions

Mononuclear phagocytes are key cells in tissue integrity and defense. Tissue-resident macrophages are abundantly present in all tissues of the body and have a complex role in ensuring tissue functions and homeostatic balance. Circulating blood monocytes can enter tissue both in steady-state conditions, for helping in replenishing the tissue-resident macrophage pool and, in particular, for acting as potent effector cells during inflammatory events such as infections, traumas, and diseases. The heterogeneity of monocytes and macrophages depends on their ontogeny, their tissue location, and their functional programming, with both monocytes and macrophages able to exert distinct or similar functions depending on the tissue-specific and stimulus-specific microenvironment. In this short review, we will review the current hypotheses on tissue-resident macrophage ontogeny and functions, as compared to blood-derived monocytes, with a particular focus on inflammatory conditions.

Monitoring Skin Dendritic Cells in Steady State and Inflammation by Immunofluorescence Microscopy and Flow Cytometry

Skin dendritic cells (DC) are strategically positioned at the body's second largest epithelial border to the environment. Hence they are the first antigen presenting cells that encounter invading pathogens and environmental antigens, including contact sensitizers and carcinogens penetrating the skin. Moreover, DC have the unique ability to induce immunity or tolerance and thus take center stage in regulating innate and adaptive immune responses. Skin DC can be divided into several phenotypically and functionally distinct subtypes. The three main subsets are Langerin⁺ epidermal Langerhans cells (LC) and Langerin⁺ as well as Langerin^neg dermal DC. In the steady state skin DC form a dense network to survey the periphery for pathogens and harmful substances breaching the cutaneous barrier. During inflammation DC become rapidly activated and start their migration to skin-draining lymph nodes where they initiate antigen-specific T cell responses. The homeostasis and mobilization of DC in the skin can be visualized by immunofluorescent staining of epidermal and dermal sheet preparations or skin sections. Here, we describe in detail how inflammation can be induced in the skin with tape stripping or FITC painting and how the skin DC network can be monitored using immunofluorescence microscopy and flow cytometry.

Monocyte-derived inflammatory Langerhans cells and dermal dendritic cells mediate psoriasis-like inflammation

Dendritic cells (DCs) have been implicated in the pathogenesis of psoriasis but the roles for specific DC subsets are not well defined. Here we show that DCs are required for psoriasis-like changes in mouse skin induced by the local injection of IL-23. However, Flt3L-dependent DCs and resident Langerhans cells are dispensable for the inflammation. In epidermis and dermis, the critical DCs are TNF-producing and IL-1β-producing monocyte-derived DCs, including a population of inflammatory Langerhans cells. Depleting Ly6C^hi blood monocytes reduces DC accumulation and the skin changes induced either by injecting IL-23 or by application of the TLR7 agonist imiquimod. Moreover, we find that IL-23-induced inflammation requires expression of CCR6 by DCs or their precursors, and that CCR6 mediates monocyte trafficking into inflamed skin. Collectively, our results imply that monocyte-derived cells are critical contributors to psoriasis through production of inflammatory cytokines that augment the activation of skin T cells.

Imiquimod exacerbates IL-23-induced skin inflammation and models psoriasis in mice. Here the authors show that this pathology is not dependent on resident dendritic cells, but on CCR6-induced immigration of monocyte-derived cells.

Duality at the gate: Skin dendritic cells as mediators of vaccine immunity and tolerance

Since Edward Jenner's discovery that intentional exposure to cowpox could provide lifelong protection from smallpox, vaccinations have been a major focus of medical research. However, while the protective benefits of many vaccines have been successfully translated into the clinic, the cellular and molecular mechanisms that differentiate effective vaccines from sub-optimal ones are not well understood. Dendritic cells (DCs) are the gatekeepers of the immune system, and are ultimately responsible for the generation of adaptive immunity and lifelong protective memory through interactions with T cells. In addition to lymph node and spleen resident DCs, a number of tissue resident DC populations have been identified at barrier tissues, such as the skin, which migrate to the local lymph node (migDC). These populations have unique characteristics, and play a key role in the function of cutaneous vaccinations by shuttling antigen from the vaccination site to the draining lymph node, rapidly capturing freely draining antigens in the lymph node, and providing key stimuli to T cells. However, while migDCs are responsible for the generation of immunity following exposure to certain pathogens and vaccines, recent work has identified a tolerogenic role for migDCs in the steady state as well as during protein immunization. Here, we examine the roles and functions of skin DC populations in the generation of protective immunity, as well as their role as regulators of the immune system.